1. Field of the Invention
The present invention relates to a third generation mobile communication, and more particularly, to a method for generating and transmitting an optimal cell (base station) identification code using Hadamard code and bi-orthogonal code in a W-CDMA mobile communication system.
2. Background of the Related Art
The Radio Access Network (RAN) standards of the Third Generation Partnership Project (3GPP) describes a Site Selection Diversity Transmit Power Control (SSDT). The SSDT, which is a selective macro diversity technique in a soft handover mode, allows a User Equipment (UE) to select one of cells in an active set called “Primary.” In this instance, all other cells are classed as “Non-primary.”
The main object of the SSDT is to transmit on the downlink from a primary cell, which reduces interference caused by multiple transmissions in a soft handover mode. A second object of the SSDT is to achieve fast site selection without intervention from networks such as UMTS Terrestrial Radio Access Network (UTRAN), thus maintaining an advantage of the soft handover. In order to select a primary cell, a temporary identification (ID) is assigned to every active cell having a transmission power level higher than a predetermined level, and the UE periodically informs a primary cell ID to other connected cells. In this instance, the UE receives, measures, an compares power levels of common pilots from the respective active cells to select the primary cell, which has the greatest pilot power. Thereafter, the UE cuts off powers from all other remaining cells which are non-primary cells.
Referring to FIG. 1, an ID code for the primary cell is transmitted to cells in the active set through a Feed-Back Indicator (FBI) field among fields of a control channel, such as an up-link Dedicated Physical Control Channel (DPCCH). Next, as can be known from FIG. 2, the FBI is transmitted by 1 or 2 bits in one slot. If the FBI is transmitted by 1 bit, one radio frame is transmitted by 15 bits, and if the FBI is transmitted by 2 bits, one radio frame is transmitted by 30 bits, because one radio frame is transmitted by 15 time slots. When an ID code is transmitted to a selected primary cell, the UE determines whether one bit is inserted or 2 bits are inserted in the FBI field per one slot before transmission.
For reference, in FIG. 1, k is related to a Spreading Factor (SF) in a dedicated physical channel, wherein the SF having a value from 256 to 4 is expressed as 256/2k. Also, the numbers of bits of fields per a slot on dedicated physical data channel and the dedicated physical control channel of uplink DPCH are defined as in the following Tables.
TABLE 1channelchannelnumberslotbitsymbolbitsbitsofformatraterateperperNdata#IkbpskspsSFframeslotbits01515256150101013030128300202026060646004040312012032120080804240240162400160160548048084800320320696096049600640640
TABLE 2No.No.No.No.slotofofofoffor-CHCHbitsbitsbitsbitsbitsbitsmatbitsymbolperperofofofof#IraterateSFframeslotNpilotNTFCINFBINTPC015152561501062021151525615010800221515256150105212315152561501070224151525615010602251515256150105221
The NFBI, representing a number of bits per slot inserted in the FBI field, is used in a closed loop mode transmit diversity or SSDT, which requires feed back between access points of the UE and the UTRAN. As shown in FIG. 2, the NFBI has an S field and a D field. The S field is used for processing a SSDT signal and the D field is used for processing a feed back mode transmission diversity signal. A length of the S field or D field may be 0, 1, 2. When both a power control by SSDT and a transmission diversity in feed back mode are used at the same time, the S field and the D field are required to each have one bit.
The operation of the SSDT for reducing interference caused by multiple transmission in the soft handover mode will next be explained in more detail.
In a soft handover mode, the SSDT is initially activated by UTRAN based on the cells in the active set, and thereafter the UTRAN of the SSDT option activated during a present soft handover period informs the cell and the UE. In this instance, a temporary ID is assigned to each active cell based on an order of the active set, and the assignment information on the IDs is given to the active cells and the UE. Thus, an active cell can identify its entry position in the active list, by which the ID code can also be determined. At the same time, the UE receives the active list and can assign ID codes of to active cells based on the order of entry of the cells on the list. Therefore, the UTRAN and the UE have corresponding information of the ID codes and the cells.
Moreover, the active list is renewed frequently and the renewed active list is provided to all active cells and the UE. After the SSDT and UE acknowledgement are activated, the UE starts to transmit the ID code of the primary cell, and the active cells start to detect primary cell ID information following a successful SSDT activation and admission of the UE acknowledgement.
The setting of temporary cell ID will next be explained.
A temporary ID is provided to cells during the SSDT for use as a site selection signal. In the SSDT mode, when an upper layer decides to make transmissions between a UE and a cell, the UE designates the most appropriate cell in the active cells as the primary cell and informs the UTRAN through the FBI field. Also, transmitting a signal to only one cell during the active SSDT mode can enhance a cell performance as intercellular interferences are reduced from the remaining other cells. The temporary cell ID is given as a binary bit sequence having a particular bit length, which is shown in Tables 3 and 4, below.
Table 3 shows temporary ID codes when the FBI is transmitted by one bit per slot, and Table 4 shows temporary ID codes when the FBI is transmitted by two bits per slot. As can be known from Tables 3 and 4, the temporary ID code may be in three forms of “Long,” “Medium,” and “Short,” and there can be 8 codes for each of the forms. The temporary ID codes are required to be transmitted within one frame. If the space for sending a given ID code cannot be obtained within a frame, i.e. the entire ID cannot be transmitted within a frame but must be split over two frames, the last bit(s) of the temporary ID code is (are) punctured. In Table 3 and 4 below, the bit(s) to be punctured in such cases are shown with brackets.
TABLE 3ID codeID labelLongMediumShortA0000000000000000000000(0)00000B1111111111111111111111(1)11111C000000001111110000111(1)00011D1111111100000001111000(0)11100E0000111111110000011110(0)00110F1111000000001111100001(1)11001G0011110000111100110011(0)01010H1100001111000011001100(1)10101
In Table 3, the long ID code with a code length 15 has a maximized minimum hamming distance dmin max. 7; the medium ID code with a code length 8 has a maximized minimum hamming distance dmin max. 4; the ID code with a code length 7, having the last bit punctured in the medium ID code with a code length 8, has a maximized minimum hamming distance dmin max. 3; and the short ID code with a code length 5 has a maximized minimum hamming distance dmin max. 2.
TABLE 4ID codeID labelLongMediumShortA0000000(0)000(0)0000000000(0)000(0)000B1111111(1)111(1)1111111111(1)111(1)111C0000000(1)000(0)0001111111(1)111(1)111D1111111(1)111(1)1110000000(0)000(0)000E0000111(1)001(1)0011111000(0)110(0)100F1111000(0)110(0)1100000111(1)001(1)011G0011110(0)011(0)0100011110(0)011(0)010H1100001(1)100(1)1011100001(1)100(1)101
In the Table 4, the long ID code with a code length 16 has a maximized minimum hamming distance dmin max. 8; the ID code with a code length 14, having the last bit pair punctured in the long ID code with a code length 16, has a maximized minimum hamming distance dmin max. 6; the medium ID code with a code length 8 has a maximized minimum hamming distance dmin max. 4; the ID code with a code length 6, having the last bit punctured in the medium ID code with a code length 8; has a maximized minimum hamming distance dmin max. 2; and the short ID code with a code length 6 has a maximized minimum hamming distance dmin max. 2.
The following Table 5 shows a number of times of site selection permitted to select a primary cell per one frame for different forms of ID codes, according to the temporary ID code characteristics shown in Tables 3 and 4.
TABLE 5codea number of FBI bits assigned per slot for SSDTlength12ALong@one site selection pertwo site selection perframeframeAMedium@two site selection perfour site selection perframeframeAShort@three site selection perfive site selection perframeframe
Referring to Table 5, if the number of FBI bits per slot is 1, the number of site selection per frame is one using a long ID code because 15 bits, one bit per slot, can be transmitted in each frame. If the number of FBI bits per slot is 2, the number of site selection per frame is two because 30 bits, two bits per slot, can be transmitted in each frame. Similarly, if the number of FBI bits per slot is 1, the number of site selection per frame is 2 using a medium ID code and 3 using a short ID code. Finally, if the number of FBI bits per slot is 2, the number of site selection per frame is 4 using a medium ID code and 5 using a short ID code.
Furthermore, when two bits of FBI are transmitted per slot, the 8 long ID codes of 14 bit lengths have a maximum cross-correlation value of “2” and a maximized minimum hamming distance 6 (dmin=6) while the 8 medium ID codes of 6 bit lengths and the 8 short ID codes of 6 bit lengths respectively have a maximum cross-correlation value of “2” and a maximized minimum hamming distance 2 (dmin=2). Here, for the long ID code of 14 bits, 2 bits of the last column of Table 2 are puncture, and for the medium ID code with a code length of 6, 2 bits of the last column are also punctured.
As described before, when the UE assigns one of the temporary ID codes as the primary cell ID code and sends the primary cell ID code after the SSDT and the UE acknowledgement, the primary cell ID code is sent through the FBI field in the uplink control channel. Particularly, a cell is non-primary if the following three conditions are met, where NID is a length (a number of bits) of the generated temporary ID.                1. The received primary ID code does not match with its own ID code;        2. the received uplink signal quality satisfies a quality threshold defined by UTRAN; and        3. the number of symbols punctured in the uplink compressed mode is smaller than λNID/3μ.        
Otherwise, if any one of the above three conditions are not met, a cell is maintained as a primary cell.
The termination of the SSDT is determined by the UTRAN. Particularly, the UTRAN terminates the SSDT in a method identical to a procedure for terminating the soft handover, and informs the termination to all cells and the UE. Thus, in the related art SSDT, performance of cell ID code used for identifying respective cells is dependent on the maximum cross-correlation value or the maximized minimum hamming distance. Accordingly, an optimal cell ID code which has a maximum cross-correlation value or a maximized minimum hamming distance is required, and a method for identifying a cell using the optimal cell ID code is required.